Copolymers containing N-vinyllactam derivatives, preparation methods thereof and photoresists therefrom

ABSTRACT

Copolymers containing N-vinyllactam derivatives protected at 3-position are provided and represented by the following formula. The copolymers are used as a photoresist material suitable for deep uv process so that high sensitivity and resolution can be obtained. In addition, ultrafine circuits can be formed and an exceptional improvement in PED stability can be accomplished by use of the photoresist. ##STR1##

BACKGROUND OF THE INVENTION

1. Field of the invention

The present invention relates to copolymers containing N-vinyllactamderivatives for use in microlithography, methods for preparing the sameand photoresists prepared from the same. More particularly, the presentinvention is concerned with copolymers containing N-vinyllactamderivatives protected at 3-position, methods for preparing thecopolymers, and photoresists suitable for deep uv exposure, which takesadvantage of the radiation sensitivity of the copolymer to form a reliefimage of high sensitivity and resolution by use of deep uv.

2. Description of the Prior Art

Generally, a photoresist comprises an alkali-soluble phenol- (orcresol-) formaldehyde novolak resin and a substituted naphthoquinonediazine compound as a photosensitive material (photoactive ingredient),as reported in U.S. Pat. Nos. 3,666,473, 4,115,128 and 4,173,470.

While the novolak resin used in such photoresist is dissolved in anaqueous alkali solution, the naphthoquinone photosensitive material actsas a dissolution inhibitor of the resist. However, when a substratecoated with the photoresist is selectively subjected to chemicalradiation, the photosensitive agent is induced to be suffered from sucha structural modification that the photoresist coating is moresolubilized by alkali at exposed region than at unexposed region. Byvirtue of such difference in solubility, a relief pattern can be formedon the coating of the substrate. That is, when the substrate is immersedin an alkaline developing solution, while the unexposed region is littleaffected, the exposed region of the photoresist coating is dissolved,forming a pattern. However, the above-mentioned novolak type resistswere found to be unsuitable to the stepper utilizing shorter wavelength,which will be described later, because they show high optical absorbancein a range of deep ultraviolet light, 200 to 300 nm.

In order to accomplish high sensitivity in the lithography process ofsemiconductor manufacture, chemical amplification resist has recentlybeen developed. Indeed, the chemical amplification resist has been inthe limelight since it was found to have the capacity to increasesensitivity 100-fold more comparing with conventional positive novolakresists. A chemical amplification resist, which takes advantage of thephotoacid generator, is generally prepared by formulating the photoacidgenerator in a matrix polymer of a structure sensitively reacting to theacid. For the mechanism of the photoreaction, when the photoacidgenerator is exposed to light or irradiated by a high energy beam, suchas X-ray beams, and electron beams, protons and strong Bronsted acid,are generated, causing the main chain or the side chain of the matrixpolymer to react toward decomposition, crosslinking or a large change inpolarity. This action of the acid induces, at the irradiated region, thesolubility therein in the given developing solution to be altered, thatis, increased or decreased. As a result, fine patterns can be formed.

As the photoacid generators, onium salts which are able to respond tolight and radiation are known. Typical onium salts are ammonium salts,oxonium salts and sulfonium salts. Recently, it has been reported thatorganic sulfonic ester can function as a photoacid generator.

Available for the matrix polymer which can react with acid are thosewhich are substituted a side chain which can be decomposed intocarboxylic acid, phenol or alcoholic functional group by acid.T-Butylester, t-butylcarbonate, t-butoxy and t-butoxycarbonyl groups areknown as the suitable substituents. Among these groups, t-butoxycarbonylgroup is found to be best in sensitivity.

Such an acid-reactive polymer in a protected state or prior to reactionwith acid, can be dissolved in organic solvent but not in alkali aqueoussolution. However, if the acid-reactive polymer is deprotected byreaction with acid, it is soluble in alkali aqueous solution because itspolarity is largely changed.

Taking advantage of this principle, the development of chemicalamplification resists has been a controversial hot issue in recentyears. T-Butoxycarbonyl-protected polyvinylphenol is reported to be oneof the possible resins, as introduced in U.S. Pat. Nos. 4,491,628,4,405,708 and 4,311,782.

A recent trend in submicrolithography is to use deep uv (wavelength 200to 300 nm) as a light source, preferably, a KrF excimer laser of highpower (wavelength 248), rather than conventional uv, e.g. g-line(wavelength 436 nm) or i-line (wavelength 365 nm), in order toaccomplish high sensitivity and resolution. However, the chemicalamplification resists are readily contaminated by the base materialspresent in air, raising a problem of stability in the post-exposuredelay (hereinafter referred to as "PED") at which T-top is formed on thecourse of fine pattern formation.

Various methods have been suggested to improve the PED stability. Amongthese methods, use of base additives (mainly amines) was found to bringan improvement in PED stability, but was disadvantageous in that itcaused a decrease in radiation-sensitivity of photoresist and the baseadditives were not uniformly distributed in film because of theirdiffusion into the film surface during processing.

SUMMARY OF THE INVENTION

Intensive research performed by the present inventors aiming to developa photoresist for use in submicrolithography which is much improved inPED stability resulted in the finding that the chemical amplificationresist polymerized with N-vinyllactam derivatives and styrenic oracrylate derivatives has high glass transition temperatures necessaryfor processing procedure, and is easily deprotected with littleabsorption of deep uv in addition to being superior in PED stability.

It is a principal object of the present invention to provide a copolymercontaining a N-vinyllactam derivative which is blocked at 3-position bya protecting group.

It is another object of the present invention to provide a method forpreparing the copolymer.

It is a further object of the present invention to provide a photoresistprepared from the copolymer.

DETAILED DESCRIPTION OF THE INVENTION

The present invention related to copolymers containing N-vinyllactamderivatives which are substituted with a protecting group at 3-position,represented by the following chemical formula I: ##STR2## wherein, R₁ ishydrogen, an alkyl group containing 1 to 10 carbon atoms, an aryl groupcontaining 6 to 12 carbon atoms or a trialkylsilyl group containing 3 to9 carbon atoms;

R₂ and R₃ independently represent hydrogen, an alkyl group containing 1to 10 carbon atoms, an aryl group containing 6 to 12 carbon atoms, atrialkylsillyl group containing 3 to 9 carbon atoms, --OR', --SO₃ R',--CO₂ R', --PO₃ R', --SO₂ R'or PO2R'wherein R'is an alkyl groupcontaining 1 to 10 carbon atoms, cycloalkyl, a cyclic group containing aheteroatom such as N, O, P and S, or an aryl group containing 6 to 12carbon atoms;

R₄ and R₅ independently represent --OH, --OR wherein R is an alkyl groupcontaining 1 to 10 carbon atoms or an aryl group containing 6 to 12carbon atoms, or the same with R₁ ;

R₇ and R₉ independently represent hydrogen, an alkyl group containing 1to 10 carbon atoms, an aryl group containing 6 to 12 carbon atoms, or atrialkylsillyl group containing 3 to 9 carbon atoms;

R₆ and R₈ independently represent an aryl group containing 6 to 20carbon atoms or an acrylate --COOR"(wherein R" is an alkyl groupcontaining 1 to 10 carbon atoms or an aryl group containing 6 to 12carbon atoms);

m is an integer of 0 to 10;

j is a molar ratio ranging from 0.005 to 0.7; and

k and 1, which may be the same or different, each are a molar ratioranging from 0.05 to 0.9.

The N-vinyllactam derivatives of the present invention are representedby the following chemical formula II: ##STR3## wherein, R₁ is hydrogen,an alkyl group containing 1 to 10 carbon atoms, an aryl group containing6 to 12 carbon atoms or a trialkylsilyl group containing 3 to 9 carbonatoms;

R₂ and R₃ independently represent hydrogen, an alkyl group containing 1to 10 carbon atoms, an aryl group containing 6 to 12 carbon atoms, atrialkylsilyl group containing 3 to 9 carbon atoms, or --OR', --SO₃ R',--CO₂ R', --PO₃ R', --SO₂ R' or PO₂ R' wherein R' is an alkyl groupcontaining 1 to 10 carbon atoms, cycloalkyl a cyclic group containing aheteroatom such as N, O, P and S, or an aryl group containing 6 to 12carbon atoms;

R₄ and R₅ independently represent --OH, --OR wherein R is an alkyl groupcontaining 1 to 10 carbon atoms or an aryl group containing 6 to 12carbon atoms, or the same with R₁ ; and

m is an integer of 0 to 10.

A copolymer in the present invention is prepared by polymerizingN-vinyllactam derivatives with styrenic or acrylate derivatives.

Concrete examples of the N-vinyllactam derivatives used in the presentinvention include 1-vinyl-2-pyrrolidinone,1-vinyl-4-butyl-2-pyrrolidinone, 1-vinyl-4-propyl-2-pyrrolidinone,3-(t-butoxycarbonyl)-1-vinyl-2-pyrrolidinone,3-(t-butoxycarbonyl)-1vinyl-4-butyl-2-pyrrolidinone,3-(t-butoxycarbonyl)-1-vinyl-4-propyl-2-pyrrolidinone,3-(tetrahydropyranyloxycarbonyl)-1-vinyl-5-ethyl-2-pyrrolidinone,1-vinyl-4-methyl-2-piperidone, 1-vinyl-5ethyl-2-piperidone,3-(t-butoxycarbonyl)-1-vinyl-4methyl-2-piperidone,3-(t-butoxycarbonyl)-1-vinyl-4-propyl-2-piperidone,1-vinyl-2-caprolactam, 1-vinyl-4-butyl-2-caprolactam,3-(t-butoxycarbonyl)-1-vinyl-2-caprolactam,3-(t-butoxycarbonyl)-1-vinyl-4-butyl-2-caprolactam,3-(t-butoxycarbonyl)-1-vinyl-6-methyl-2-caprolactam,3-(tetrahydropyranyloxycarbonyl)-1-vinyl-5-butyl-2-caprolactam,3-tetrahydropyranyloxycarbonyl)-1-vinyl-6-propyl-2-caprolactam,3-(tetrahydrofuranyloxycarbonyl)-1-vinyl-2-pyrrolidinone,3-(tetrahydrofuranyloxycarbonyl)-1-vinyl-4-butyl-2-pyrrolidinone, and3-(tetrahydropyranyloxycarbonyl)1-vinyl-6-butyl-2-caprolactam.

For copolymer, another monomer is used the examples of which include4-(t-butoxycarbonyloxy)-1-vinylcyclohexane,3,5-(di-t-butoxycarbonyloxy)-1-vinylcyclohexane,4-(tetrahydropyranyloxy)-1-vinylcyclohexane,4-(tetrahydrofuranyloxy)-1-vinylcyclohexane,3,5-(ditetrahydropyranyloxy)-1-vinylcyclohexane,3,5-(ditetrahydrofuranyloxy)-1-vinylcyclohexane,t-butoxycarbonyloxystyrene, styrene, tetrahydropyranyloxystyrene,4-hydroxystyrene, 4-acetoxystyrene, 3-methyl-4-hydroxystyrene, t-butylacrylate, t-butyl methacrylate, adamantylacrylate, andadamantylmethacrylate.

The copolymers of the present invention may be obtained in bulkpolymerization or in a solution polymerization. As a solvent forpolymerization, cyclohexanone, methylethylketone, benzene, toluene,dioxane, dimethylformamide alone or a combination thereof may beselected. Usually, the polymerization is carried out in the presence ofa polymerization initiator, such as benzoylperoxide,2,2'-azobisisobutyronitrile (AIBN), acetyl peroxide, lauryl peroxide, ort-butylperacetate.

A copolymer of hydroxystyrene, t-butylacrylate and3-(t-butoxycarbonyl)-1-vinyl-2-caprolactam (hereinafter referred to as"BCVC"), one of the copolymers of the present invention,poly(4-hydroxystyrene-co-t-butyl acrylate-co-BCVC) (hereinafter referredto as "poly(HOST-co-TBA-co-BCVC"), was found to be highly transparent asproven in the experiment in which a film 1 μm thick showed an opticalabsorbance of 0.12 or lower at deep uv range (200 to 300 nm). Thermalgravity analysis (TGA) showed that poly(HOST-co-TBA-co-BCVC) was stableat up to 180° C. At higher than 180° C., rapid deprotection oft-butoxycarbonyl group occurs, producing 2-methylpropene and CO2 In thepresence of acid, the t-butoxycarbonyl group was rapidly deprotectedinto 2-methylpropene and CO₂ even at 135° C. This fact means thatpoly(HOST-co-TBA-co-BCVC) is far superior in thermal property by virtueof its high thermal decomposition temperature and is readily deprotectedat low temperatures in the presence of acid. Differential scanningcalorimetry (DSC) shows that the glass transition temperature ofpoly(HOST-co-TBA-co-BCVC) is about 165° C.

Besides the transparency to deep uv and the ease of deprotection, agreat improvement in PED stability is expected for a novel photoresistcontaining the copolymer of the present invention by virtue ofappropriate photosensitivity and the introduction of the base unit(BCVC). All of the polymers of N-vinyllactam derivatives which areprotected at 3-position, are high in sensitivity and contrast.Particularly, poly(HOST-co-TBA-co-BCVC), when BCVC is contained at anamount of 1 mole %, shows an appropriate sensitivity of about 10 mJ/cm²and a high contrast. As the mole % of BCVC increases, the sensitivitydecreases. For example, when BCVC is present in an amount of 2.6 mole %,the resulting copolymer shows a sensitivity of 21 mJ/cm². However, thisdecrease in sensitivity is much less serious than those obtainable fromthe addition of conventional amine base additives.

Ordinary experiment for fine pattern formation confirmed that thecopolymers prepared from the N-vinyllactam derivatives and styrenic oracrylate derivatives according to the present invention could be appliedfor high sensitive chemical amplification resists.

The thermal decomposition behavior analysis of the polymers was carriedout in nitrogen atmosphere at a temperature elevation of 10° C./minusing DSC, commercially available from Perkin Elmer, identified as MODEL7, and TGA. The inherent viscosities of the polymers were determined inthe state of dimethylformamide solution of 0.5 g/dl at 25° C. by use ofa glass capillary tube.

A better understanding of the present invention may be obtained in lightof the following examples which are set forth to illustrate, but are notto be construed to limit, the present invention.

EXAMPLE I Synthesis of 3-(t-Butoxycarbonyl)-1-vinyl-2-pyrrolidinonemonomer

To a solution of diisopropylamine 14 ml (100 mmol) in tetrahydrofuran 40ml free of moisture, 40 ml (100 mmol) of 2.5 M n-butyllithium was added,and the resulting solution was stirred at -78° C. for 30 min and allowedto react until the temperature was elevated up to room temperature.After being freezed down to -78° C., the solution was added with 13.9 g(100 mmol) of N-vinylpyrrolidinone and subjected to reaction at the sametemperature for 30 min. Thereafter, 24 g (110 mmol) of di-t-butyldecarbonate was added dropwise, followed by the reaction at -78° C. for2 hours. This solution was diluted with diethyl ether and washed manytimes with pure water. The organic solvent of the organic phase wasevaporated and the residue was subjected to silica gel columnchromatography, to obtain 15 g of pure3-(t-butoxycarbonyl)-1-vinyl-2-pyrrolidinone (hereinafter referred to as"BCVP"). Its chemical structure was determined by IR spectra and NMR.

EXAMPLE II Synthesis of 3-(t-Butoxycarbonyl)-1-vinyl-2-caprolactammonomer

To a solution of diisopropylamine 14 ml (100 mmol) in tetrahydrofuran 40ml free of moisture, 40 ml (100 mmol) of 2.5 M n-butyllithium was added,and the resulting solution was stirred at -78° C. for 30 min and allowedto react until the temperature was elevated up to room temperature.After being freezed down to -78° C. the solution was added with 13.9 g(100 mmol) of N-vinylcaprolactam and subjected to reaction at the sametemperature for 30 min. Thereafter, 24 g (110 mmol) of di-t-butyldecarbonate was added dropwise, followed by the reaction at -78° C. for2 hours. This solution was diluted with diethylether and washed manytimes with pure water. The organic solvent of the organic phase wasevaporated and the residue was subjected to silica gel columnchromatography, to obtain 17.2 g of pure3-(t-butoxycarbonyl)-1-vinyl-2-caprolactam (hereinafter referred to as"BCVC").

IR spectra and NMR analysis were taken to determine the chemicalstructure of the BCVC synthesized.

EXAMPLE III Synthesis of3-(Tetrahydropyranyloxycarbonyl)-1-vinyl-2-pyrrolidinone monomer

10.6 g (0.05 mol) of the BCVP synthesized in Example I was dissolved in50 ml of tetrahydrofuran free of moisture. To this solution 4.3 g (0.05mol) of tetrahydropyran and 0.3 g of p-toluene sulfonic acid were added,and allowed to react at 0° C. for 4 hours. The solution was diluted withdiethylether and washed many times with pure water. The organic solventof the organic phase was evaporated and the residue was subjected tosilica gel column chromatography, to obtain 9.8 g of. pure3-(tetrahydropyranyloxycarbonyl)-1-vinyl-2-pyrrolidinone (hereinafterreferred to as "TPVP"). Its chemical structure was determined by IRspectra and NMR.

EXAMPLE IV Synthesis of3-(Tetrahydrofuranyloxycarbonyl)-1-vinyl-2-pyrrolidinone monomer

8.9 g of 3-(t-butoxycarbonyl)-1-vinyl-2-pyrrolidinone (BCVP) wasdissolved in 50 ml of tetrahydrofuran free of moisture. To this solution1.0 g of sodium hydride and 4.4 g of 2-chlorotetrahydrofuran were addedand allowed to react at room temperature for 1 hour. This solution wasdiluted with diethylether and washed many times with pure water. Theorganic solvent of the organic phase was evaporated and the residue wassubjected to silica gel column chromatography, to obtain 9.3 g of pure3-(tetrahydrofuranyloxycarbonyl)-1-vinyl-2-pyrrolidinone (hereinafterreferred to as "TFVP"). Its chemical structure was determined by IRspectra and NMR.

EXAMPLE V Synthesis of3-(Tetrahydropyranyloxycarbonyl)-1-vinyl-2-caprolactam monomer

10.7 g of pure 3-(tetrahydropyranyloxycarbonyl)-1-vinyl-2-caprolactam(hereinafter referred to as "TPVC") was synthesized in a similar mannerto that of Example III, except for using 6.7 g of N-vinylcaprolactaminstead of BCVP.

IR spectra and NMR analysis were taken to determine the chemicalstructure of TPVC synthesized.

EXAMPLE VI Synthesis of3-(Tetrahydrofuranyloxycarbonyl)-1-vinyl-2-caprolactam monomer

The procedure of Example IV was repeated using 10.3 g of3-(t-butoxycarbonyl)-1-vinyl-2-caprolactam (BCVC), to obtain pure 10.8 gof pure 3-(tetrahydrofuranyloxycarbonyl)-1-vinyl-2-caprolactam(hereinafter referred to as "TFVC").

IR spectra and NMR analysis were taken to determine the chemicalstructure of TFVC synthesized.

EXAMPLE VII Synthesis of poly(HOST-co-TBA-co-BCVC) Copolymer

2.65 g of BCVC monomer synthesized in Example II, 6.0 g ofacetoxystyrene and 3.31 g of t-butyl acrylate were dissolved in tolueneand placed in a polymerization glass ample. The reactants werepolymerized at 65° C. for 22 hours under reduced pressure in thepresence of 0.2 mole of AIBN, a polymerization initiator. The polymerwas precipitated in petroleum ether and the precipitates were dried togive 8.4 g of poly(ACOST-co-TBA-co-BCVC) copolymer: yield 71%. It wasadded in a methanol solution containing 12 ml of 28% ammonia water andsubjected to base hydrolysis at room temperature for 10 hr withstirring. Neutralization with acetic acid, precipitaion in distilledwater, and drying were sequentially carried out to givepoly(HOST-co-TBA-co-BCVC). Its inherent viscosity was determined in thestate of dimethylformamide solution at 25° C. by use of a ubbelohe typeviscometer. The results are given as shown in Table 1 below.

Other copolymers were prepared by using the combinations suggested inTable 1 below. The molar ratios of the units in the copolymers weremeasured by NMR analysis and thermoanalysis and given in Table 1, alongwith their viscosities.

                                      TABLE 1                                     __________________________________________________________________________        Mole Ratio       Inherent                                                                           Mole Raios of                                          of Monomers  Viscosity Unit in Polymer                                     Polymer                                                                           ACOST:                                                                             TBA:                                                                             BCVC                                                                              Yield (%)                                                                          (dL/g)                                                                             ACOST:                                                                             TBA:                                                                             BCVC                                        __________________________________________________________________________    A   10   7  0   82   0.53 62   38 0                                             B 10 6 0.5 80 0.51 64.3 35.0 0.7                                              C 10 6 1 78 0.49 63.6 35.0 1.4                                                D 10 7 3 71 0.46 58.1 39.3 2.6                                              __________________________________________________________________________

EXAMPLES VIII THROUGH XIV

Using N-vinylpyrrolidinone (NVP), N-vinylcaprolactam (NVC), BCVP(Example I), TPVP (Example III), TFVP (Example IV), TPVC (Example V) andTFVC (Example VI), the procedure of Example VII was repeated to giverespective copolymers. The results are shown in Table 2 below.

                                      TABLE 2                                     __________________________________________________________________________        Mole Ratio       Inherent                                                                           Mole Raios of                                          of Monomers  Viscosity Unit in Polymer                                     Polymer                                                                           ACOST:                                                                             TBA:                                                                             BCVC                                                                              Yield (%)                                                                          (dL/g)                                                                             ACOST:                                                                             TBA:                                                                             BCVC                                        __________________________________________________________________________    8   10   6  3(A)                                                                              84   0.43 60.2 37.0                                                                             2.8                                           9 10 6 3(B) 82 0.51 59.3 38.0 2.7                                             10 10 6 3(C) 78 0.45 64.1 34.0 1.9                                            11 10 6 3(D) 75 0.47 60.9 36.7 2.4                                            12 10 6 3(E) 80 0.50 59.3 39.0 1.7                                            13 10 6 3(F) 78 0.48 63.1 35.0 1.9                                            14 10 6 3(G) 77 0.48 59.7 38.7 1.6                                          __________________________________________________________________________     .sup.a A:NVP, B:NVC, C:BCVP, D:TPVP, E:TFVP, F:TPVC, G:TFVC              

EXAMPLE XV Preparation of Resist Solution and Formation of Positive FinePattern

10 to 30 % by weight of poly(HOST-co-TBA-co-BCVC) synthesized in ExampleVII was dissolved in propylene glycol methyl ether acetate. In thissolution, an onium salt or organic sulfonic acid, acting as a photoacidgenerator, was added at an amount of 5.0 to 30.0 % by weight based onthe weight of the resist polymer. Filtration with ultrafine filter gavea chemical amplification resist solution. Subsequently, it wasspin-coated on a silicon wafer, to form a thin film about 1.0 μm thick.This wafer was pre-baked in an oven or hot plate at 120° C. for 1-5 min,exposed to the light radiated from a deep uv contact printer or KrFexcimer laser stepper, subjected to post exposure-baking (PEB) in anoven or hot plate at 130 to 150° C. for 1-5 min, and immersed in analkaline water for 90 seconds for development. As a result, a positiveresist pattern of 0.2 μm 1/s was obtained.

Prior to PEB, the exposed resist film was allowed to stand in air for atest for PED stability. Even after 4 hrs, a fine pattern without a T-topformation was obtained.

As described and proven hereinbefore, the novel copolymers which areable to be used as chemical amplification resists suitable for deep uvare successfully synthesized according to the present invention. Inaddition, the photoresist made of the polymers according to the presentinvention is of high sensitivity so that patterns can be formed withhigh resolution.. Furthermore, the introduction of N-vinyllactamderivative brings about a great enhancement in PED stability. Therefore,the radiation-sensitive polymers can be applied for highly integratedsemiconductor devices and electronic device lithography. Consequently,ultrafine circuits can be formed and an exceptional improvement inpattern formation can be attained by using the photoresist preparedaccording to the present invention.

The present invention has been described in an illustrative manner, andit is to be understood that the terminology used is intended to be inthe nature of description rather than of limitation.

Many modifications and variations of the present invention are possiblein light of the above teachings. Therefore, it is to be understood thatwithin the scope of the appended claims, the invention may be practicedin ways other than those specifically described.

What is claimed is:
 1. A phototresist copolymer comprising:N-vinyllactamderivative blocked by a protecting group represented by the followinggeneral Formula II: ##STR4## wherein: R₁ is hydrogen, an alkyl groupcontaining 1 to 10 carbon atoms, or a trialkylsilyl group containing 3to 9 carbon atoms; R₂ is a protecting group; R₃ is H or the same as R₂ ;R₄ and R₅ independently represent --OH, or --OR wherein R is an alkylgroup containing 1 to 10 carbon atoms or the same as R₁ ; and m is aninteger of 0 to 10; wherein the protecting group is decomposed by anacid to be a functional group soluble in an alkaline developer.
 2. Aphotoresist copolymer according to claim 1 wherein said functional groupsoluble in an alkaline developer is carboxylic acid, phenol or alcoholicgroup.
 3. A photoresist copolymer according to claim 1 wherein saidprotecting group is selected from the group consisting of: an alkylgroup containing 1 to 10 carbon atoms; a trialklylsilyl group containing3 to 9 carbon atoms; --OR', --SO₃ R', --CO₂ R', --PO₃ R', --SO₂ R', and--PO₂ R' wherein R' is an alkyl group containing 1 to 10 carbon atoms,cycloalkyl or a cyclic group containing a heteroatom selected from thegroup consisting of N, O, P and S.
 4. A photoresist copolymer accordingto claim 1 wherein said protecting group is selected from the groupconsisting of t-butylester, t-butylcarbonate, t-butoxy,t-butoxycarbonyl, tetrahydropyranyloxycarbonyl andtetrahydrofuranyloxycarbonyl.
 5. A photoresist copolymer according toclaim 1 wherein said N-vinyllactam derivative is selected from the groupconsisting of:3-(t-butoxycarbonyl)-1-vinyl-2-pyrrolidinone (BCVP);3-(t-butoxycarbonyl)-1-vinyl-4-butyl-2-pyrrolidinone;3-(t-butoxycarbonyl)-1-vinyl-4-propyl-2-pyrrolidinone;3-(tetrahydropyranyloxycarbonyl)-1-vinyl-2-pyrrolidinone (TPVP);3-(tetrahydropyranyloxycarbonyl)-1-vinyl-5-ethyl-2-pyrrolidinone;3-(tetrahydrofuranyloxycarbonyl)-1-vinyl-2-pyrrolidinone (TFVP);3-(tetrahydrofuranyloxycarbonyl)-1-vinyl-4-butyl-2-pyrrolidinone;3-(t-butoxycarbonyl)-1-vinyl-4-methyl-2-piperidone;3-(t-butoxycarbonyl)-1-vinyl-4-propyl-2-piperidone;3-(t-butoxycarbonyl)-1-vinyl-2-caprolactam (BCVC);3-(t-butoxycarbonyl)-1-vinyl-4-butyl-2-caprolactam;3-(t-butoxycarbonyl)-1-vinyl-6-methyl-2-caprolactam;3-(tetrahydropyranyloxycarbonyl)-1-vinyl-2-caprolactam (TPVC);3-(tetrahydropyranyloxycarbonyl)-1-vinyl-5-butyl-2-caprolactam;3-(tetrahydropyranyloxycarbonyl)-1-vinyl-6-propyl-2-caprolactam;3-(tetrahydropyranyloxycarbonyl)-1-vinyl-6-butyl-2-caprolactam;3-(tetrahydrofuranyloxycarbonyl)-1-vinyl-2-caprolactam (TFVC).
 6. Aphotoresist copolymer according to claim 1 wherein said copolymerfurther comprises styrenic or acrylate derivatives.
 7. A photoresistcopolymer according to claim 6 wherein said styrenic and acrylatederivative independently contains hydroxylalkyl group(s) or protectinggroup(s).
 8. A photoresist copolymer according to claim 6 wherein saidstyrenic derivative is hydroxystyrene (HOST).
 9. A photoresist copolymeraccording to claim 6 wherein said acrylate derivative is t-butylacryalte(TBA).
 10. A photoresist copolymer according to claim 1, wherein saidcopolymer is represented by Formula I: ##STR5## wherein: R₁ is hydrogen,an alkyl group containing 1 to 10 carbon atoms, or a trialkylsilyl groupcontaining 3 to 9 carbon atoms;R₂ is a protecting group wherein theprotecting group is decomposed by an acid to be a functional groupsoluble in an alkaline developer; R₃ is H or the same as R₂ ; R₄ and R₅independently represent --OH, --OR wherein R is an alkyl groupcontaining 1 to 10 carbon atoms or is the same as R₁ ; and R₇ and R₉independently represent hydrogen, an alkyl group containing 1 to 10carbon atoms, or a trialkylsilyl group containing 3 to 9 carbon atoms;R₆ and R₈ independently represent an aryl group containing hydroxyalkylgroup(s) or protecting group(s); m is an integer of 0 to 10; j is amolar ratio ranging from 0.005 to 0.7; and k and l, which can be thesame or different, each have a molar ratio ranging from 0.05 to 0.9. 11.A photoresist copolymer according to claim 1, wherein said copolymer isselected from the group consisting of:Poly(HOST-co-TBA-co-BCVC);Poly(HOST-co-TBA-co-BCVP); Poly(HOST-co-TBA-co-TPVP); andPoly(HOST-co-TBA-co-TFVP.
 12. A method for preparing a photoresistcopolymer represented by the following general Formula I which comprisesthe steps of:(a) dissolving in an organic solvent (i) N-vinyllactamderivatives represented by the following general Formula II and (ii)styrenic or acrylate derivatives; and (b) adding a polymerizinginitiator into the resultant solution; ##STR6## wherein: R₁ is hydrogen,an alkyl group containing 1 to 10 carbon atoms, or a trialkylsilyl groupcontaining 3 to 9 carbon atoms; R₂ is a protecting group; R₃ is H or thesame as R₂ ; R₄ and R₅ are independently --OH, --OR wherein R is analkyl group containing 1 to 10 carbon atoms or the same as R₁ ; m is aninteger of 0 to 10; wherein the protecting group is decomposed by anacid to be a functional group soluble in an alkaline developer ##STR7##wherein: R₁ is hydrogen, or an alkyl group containing 1 to 10 carbonatoms, or a trialkylsilyl group containing 3 to 9 carbon atoms; R₂ is aprotecting group wherein the protecting group is decomposed by an acidto be a functional group soluble in an alkaline developer; R₃ is H orthe same as R₂ ; R₄ and R₅ independently represent --OH, --OR wherein Ris an alkyl group containing 1 to 10 carbon atoms or is the same as R₁ ;and R₇ and R₉ independently represent hydrogen, an alkyl groupcontaining 1 to 10 carbon atoms, or a trialkylsilyl group containing 3to 9 carbon atoms; R₆ and R₈ independently represent an aryl groupcontaining hydroxyalkyl group(s) or protecting group(s); m is an integerof 0 to 10; j is a molar ratio ranging from 0.005 to 0.7; and k and lwhich can be the same or different, each have a molar ratio ranging from0.05 to 0.9.
 13. A method according to claim 12 wherein saidpolymerization initiator is selected from the group consisting ofbenzoylperoxide, 2,2'-azobisisobutronitrile (AIBN), acetyl peroxide,lauryl peroxide, and t-butylperacetate.
 14. A method according to claim12, wherein said organic solvent is selected from the group consistingof cyclohexanone, methylethylketone, benzene, toluene, dioxane,dimethylformamide, and mixtures thereof.